The present invention relates to a method of replacing a plate in a printing press and an apparatus therefor wherein an old plate which is mounted on the circumferential surface of a plate cylinder and the two ends of which are gripped by plate lockup devices is replaced with a new plate prepared outside the plate replacing apparatus.
A gap is formed along the entire length in the outer circumference of each plate cylinder in a printing press. A plate lockup apparatus consisting of a leading-side lockup device for gripping the leading edge of the plate and a trailing-side lockup device for gripping the trailing edge of the plate is fixed on the bottom surface of the gap to extend in the axial direction of the plate cylinder. In a multicolor printing press, one plate cylinder is arranged in each printing unit, and the plate lockup device is arranged in each plate cylinder.
Each of the conventional leading- and trailing-side lockup devices comprises an elongated lockup table extending in the axial direction of the plate cylinder, a plurality of gripper plates, arranged in tandem with each other in the axial direction of the plate cylinder and swingably supported at an edge portion of this lockup table by a plurality of bolts, for gripping or releasing the plate with or from the lockup table, and a pivotal cam shaft having a plurality of cams which can be respectively engaged with gaps at the edges of the gripper plates. The plurality of cams are pivotally aligned along the axis of the plate cylinder. A plurality of compression coil springs are interposed between the lockup table and the gripper plates to bias the gripper plates in an open direction.
With the above arrangement, in order to mount a plate on a plate cylinder, when a cam shaft of the leading-side lockup device is pivoted, the gripper plates which are divided in the axial direction of the plate cylinder are released upon disengagement from the cams and are simultaneously opened by the elastic forces of the compression coil springs. An end of the plate is inserted between the leading-side lockup device and the corresponding lockup table. When the cam plate is pivoted in the direction opposite to the direction described above, the gripper plates are pivoted against the elastic forces of the compression coil springs by the behavior of the cams and are closed, thereby gripping the leading edge of the plate.
Another conventional apparatus is disclosed in Japanese Patent Laid-Open No. 1-127346. In this apparatus, the lockup tables and the gripper plates are disposed in the radial direction of a plate cylinder so that a trailing-side gripper surface of the plate conventionally formed in the circumferential direction of the plate cylinder is formed in the radial direction of the plate cylinder. The edge of the plate is bent at a right angle by an external bending machine so as to cause the gripper surfaces to grip the bent edge. With this arrangement, after the leading edge of the plate is gripped, the bent portion of the trailing edge portion of the plate wound around the circumferential surface of the plate is inserted between the lockup tables and the gripper plates. The gripper plates are swung by a cam mechanism to grip the bent portion of the plate. The trailing-side lockup device as a whole is circumferentially moved to uniformly mount the plate, thereby bringing the plate into tight contact with the surface of the plate cylinder.
In such a conventional press, when an old plate is replaced with a new plate due to changes in contents of printed matters, the trailing-side cam shaft is pivoted to open the trailing-side lockup device. One end of the plate which is released from gripping is kept held, and the plate cylinder is rotated. The leading-side cam shaft is pivoted to open the leading-side lockup device to release the other end of the plate from gripping, thereby removing the old plate. Thereafter, opening/closing of the plate lockup devices and the pivotal operation of the plate cylinder are repeated to mount the new plate.
The following two conventional methods are employed as plate replacing methods in multicolor printing presses. According to the first method, removal of an old plate and supply of a new plate are performed in units of printing units. More specifically, at the time of plate replacement, a plate cylinder of each printing unit is pivoted to a plate removal position, and the leading- and trailing-side plate lockup devices are opened. The plate cylinder is rotated in the reverse direction to remove the old plate. The plate cylinder is then pivoted to a plate supply position, and one end of the new plate is gripped by the leading-side plate lockup device. The plate cylinder is then rotated in the forward direction to wind the plate around the circumferential surface of the plate cylinder. The other end of the plate is then gripped by the trailing-side plate lockup device. The plate is then uniformly kept taut and is therefore brought into tight contact with the circumferential surface of the plate cylinder. The above operations are repeated in units of printing units, thereby completing the plate exchange operations.
According to the second method, for example, the plate removal operations are repeated from the printing unit of the fourth color to the printing unit of the first color, and the plate supply operations are then repeated from the printing unit of the first color to the printing unit of the fourth color.
In the plate replacing method and apparatus in the conventional printing press, however, pivotal movement of the cam shaft for opening/closing the plate gripper surfaces and rotation of the plate cylinder must be performed manually or upon operation of a push button. The number of operation steps is increased, workability is degraded, much labor is required. In addition, the preparation time is undesirably prolonged to degrade productivity. In particular, plate replacement is cumbersome and requires skills since the old and new plates must be manually held. In addition, the new plate cannot be placed to stand by at a mounting position of the plate cylinder during printing. The old plate must be removed from the press during the replacement. A preparation period is therefore undesirably prolonged, and productivity is degraded. In addition, automatic plate replacement cannot be performed due to a requirement of plate holding.
As described above, since the motor is connected to the drive shaft via a belt, when the plate exchange apparatus is added, the distal end portion of the plate exchange apparatus does not accurately oppose the gripper surfaces of the plate lockup devices due to slippage and contraction of the belt. A gripping failure may thus occur. If the motor and the drive shaft are connected via gears or a chain, this arrangement may pose serious safety problems.
In the conventional plate replacing method, at least a sum of the plate removal times and the plate supply times of the respective printing units is required as a total plate exchange period. The plate exchange period cannot be shortened.